Preheat oven to 425°. Lightly coat a doughnut pan with cooking spray.
Combine flours and next 4 ingredients (through salt) in a large bowl, stirring well with a whisk.
Combine buttermilk, eggs, honey, butter, and vanilla, stirring well with a whisk. Add buttermilk mixture to flour mixture; whisk just until combined.
Spoon batter into doughnut pans, filling twothirds full. Bake in middle of oven until doughnuts spring back when touched and are golden on bottom (about 8 minutes). Let cool in pan slightly (about 4 minutes); turn out. Coat with toppings (see ideas, below), if desired.
Combine 2 tablespoons fresh lemon juice and 1 1/2 cups powdered sugar, stirring well with a whisk. Dip 1 side of the cooled doughnuts into glaze; let cool on a rack, glazed side up.
Melt 6 ounces bittersweet chocolate in a bowl in microwave until smooth. Dip tops of doughnuts into chocolate and place chocolate side up on a rack to cool. Sprinkle with 1/2 cup chopped hazelnuts; let sit about 10 minutes.
Melt 4 ounces (1 cup) bittersweet chocolate in microwave until smooth. Fill a small squeeze bottle or zip-top plastic bag (snip a tiny hole in 1 corner of bag) with melted chocolate; pipe chocolate onto tops of doughnuts. Chill doughnuts until chocolate sets (about 10 minutes).
Combine 1/4 cup ground cinnamon and 1/4 cup sugar. While still warm, coat doughnuts in sugar; let cool on rack, sugar-coated side up.